Dialing signals generated by pushbutton-equipped telephone sets are generally transmitted as sine waves of different frequencies serving to indicate selected digits. In central offices operating with pulse-code modulation (PCM), the incoming voice frequencies are often sampled and digitized before being routed to their destinations. In these cases, therefore, sine waves generated by a pushbutton selector undergo the same quantizing process.
A receiver for such multifrequency-coded digitized signals has been disclosed in commonly owned U.S. Pat. No. 4,185,172 (Melindo et al).
Conventional central-office equipment designed to handle analog multifrequency signals converts incoming sine waves into square waves that can be decoded by logical circuitry which translates their frequencies into the numerical information needed for completing the call. Such equipment, however, cannot be readily utilized with quantized signal samples including the usual predetermined number of amplitude bits accompanied by a sign bit. Although a change in sign bit from one sample to the next indicates that the corresponding sine wave has undergone a zero crossing within the sampling period immediately preceding the detection of that change, a square wave with rising and falling pulse flanks occurring at the instants of the detected sign-bit changes would not be completely in step with the underlying square wave. Since the delay occurring between a zero crossing and a corresponding pulse flank could range from zero to an entire sampling period, and since that sampling period cannot be arbitrarily reduced in a PCM system, the square wave will experience significant phase shifts which may interfere with decoding.